YANGON (Reuters) - Myanmar security forces sealed off Yangon's Shwedagon Pagoda on Wednesday, fired tear gas and arrested up to 200 monks trying to get into the shrine, cracking down on the biggest anti-junta protests in nearly 20 years.

But despite a heavy deployment of troop and riot police at key points across the city, 500 monks marched towards the downtown Sule Pagoda area, the end-point of a week of peaceful protests, witnesses said.

Others joined them along the way, swelling their ranks.

Witnesses and monk sources said some of the deeply revered Buddhist clergy were beaten and manhandled by riot police taking them away from the Shwedagon Pagoda, starting point of the past week's monk-led protests against 45 years of military rule.

Witnesses said they heard no gunshots, but they said security forces burnt plastic pipes to make acrid smoke which filled the air around the pagoda.

The atmosphere at the gilded Shwedagon shrine was "very tense," one witness said, with onlookers angry at the use of violence against the maroon-clad monks.

But the numbers of monks there were far fewer than on Monday or Tuesday after troops and riot police took up positions outside at least six big activist monasteries.

There was also a heavy security presence at the Sule Pagoda, a clear sign the generals were trying to prevent more mass marches.

Hundreds of soldiers waited in a park behind Sule Pagoda, the scene of some of the worst bloodshed when troops opened fire on protesters in 1988, the former Burma's last major uprising.